Linear Pyrolysis of Ammonium Perchlorate with Additives

G. V. Ivanov, V. I. Bratinsky, A. Ya. Solov’ev

(Presented by the scientific seminar of the Department of Radiation Chemistry)

EXPERIMENTAL PART

Since, in the present case, the greatest interest lies in determining the relative change in the rate of pyrolysis under the influence of a catalyst, the simplest of the possible variants of pyrolysis was used—pyrolysis on a continuous heating plate. For the same reason, no evaluation was made of the difference between the temperature of the tablet surface at the point of contact with the plate and the temperature of the plate. The latter was measured by a thermocouple whose junction was located in a recess on the underside of the plate near the contact.

To improve the accuracy of the measurements, the thermocouple was calibrated while in its working position. Individual crystals of substances with known melting points (no larger than 0.1 mm) were placed on the plate. The onset of melting of the crystals was recorded with a microscope.

A horizontally positioned tungsten plate was heated from below by thermal radiation from a powerful electric coil. The temperature of the plate could be varied within the range of \(300^\circ\)–\(800^\circ\text{C}\). Comparative experiments showed that the pyrolysis rate did not differ significantly in those cases where the plate was covered with small sheets—

…with aluminum foil (aluminum oxide does not catalyze the decomposition of ammonium perchlorate) and when the foil was absent. Thus, the catalytic influence of the tungsten plate may be neglected.

Specimens measuring \(1 \times 8 \times 35\) mm were pressed under a specific pressure of \(5\ \text{t}/\text{cm}^2\). The catalysts were introduced into the perchlorate by mechanical mixing, in an amount of 2% by weight. The dispersity of the catalysts was the same, on the order of \(50\mu\).

The pellet was placed perpendicular to the plate and pressed against it with a small load. A simple contact device connected to the pellet holder actuated an electric stopwatch; in this way the time of gasification of prescribed sections of the pellet (15–20 mm) was determined. The time count began after 3–5 mm of the pellet had gasified or burned away.

For the selected load and pellet cross-section, as the temperature of the plate decreased, a combustion regime or a flameless gasification regime of the pellet was observed, sharply differing in rate.

The experimental results in the coordinates—reciprocal absolute temperature \((1000/T^\circ K)\) versus logarithm of the rate \((\lg U)\)—are presented in Fig. I.

Fig. I. Pyrolysis of ammonium perchlorate with additives.

The points on the curves are the averaged results of 5–10 parallel experiments.

Pure ammonium perchlorate ignites at a plate temperature above \(450^\circ\). Despite the presence of a flame between the specimen and the plate, the burning rate depends on the temperature of the latter (Fig. 2). Below \(450^\circ\), flameless gasification is observed at a rate of the order of \(10^{-2}\) mm/sec.

In the presence of 2% \(CuO\) and 2% \(MnO_2\), burning begins at a plate temperature of \(350^\circ\), and its rate, unlike that of pure perchlorate, changes almost not at all with increasing plate temperature. Below \(350^\circ\), pyrolysis proceeds at a rate of the order of \(10^{-4}\)–\(10^{-5}\) mm/sec. The nature of the influence of \(Fe_2O_3\) is different. In its presence, a comparatively smooth transition (in terms of the magnitudes of the rates) from pyrolysis to burning is observed. The pyrolysis rate increases strongly; the burning rate is retarded.

DISCUSSION OF RESULTS

In works [2, 3] two modes of pyrolysis are reported. It has been established that in the high-temperature mode (rates less than \(10^{-1}\) mm/sec), pyrolysis is limited by the destruction of the substance in the condensed phase and by diffusion of the decomposition products through the bulk and the surface.

At a certain temperature on the line \(u = f(T^\circ)\) there is a break; above this temperature (rate greater than \(10^{-1}\) mm/sec), the limiting role is already played by the processes of desorption and evaporation of the gaseous pyrolysis products.

From the conclusions of works [2, 3] it follows that the rate of the process can change only as a result of an effect: in the high-temperature mode (all the more so during burning), on processes in the gas phase; in the low-temperature mode, on processes in the condensed phase.

Experimentally, this can be confirmed by selectively affecting, for example, only the processes in the condensed phase. Such an effect may be regarded as preliminary irradiation of ammonium perchlorate. Indeed, at doses on the order of \(10^5\) rad, about 0.002% of the perchlorate undergoes radiolysis. The radiolysis products—gaseous, thermally unstable compounds—remain trapped in the lattice of the original salt. This accelerates the decomposition of the substance (see, for example, [4]); moreover, here one can hardly expect any noticeable change in the character of the processes occurring in the gas phase.

We carried out irradiation of ammonium perchlorate with X-rays on an RUP-200 installation at a dose of \(10^5\) rad.

It is seen from Fig. 1 that irradiation greatly increases the pyrolysis rate in the low-temperature regime; however, ignition occurs at the same temperature as in the case of the unirradiated sample, and preliminary irradiation does not change the burning rate at a plate temperature above \(450^\circ\).

These results are consistent with the conclusions of [2, 3]: the effect of preliminary irradiation is most strongly manifested in the low-temperature regime, when the processes in the condensed phase are rate-limiting.

It may therefore be considered that the action of additives will also be analogous, i.e., catalysis of decomposition processes in the condensed phase can change the pyrolysis rate only at temperatures below the ignition temperature. Above it, the effects of additives should be associated with catalysis of processes in the gas phase. On the basis of what has been said, and taking into account the results of [1] (and the references to other sources given there), the effects of additives may be explained as follows.

Copper and manganese oxides are catalysts of ammonia oxidation. In their presence, oxidation (ignition) apparently is possible already at \(300^\circ\), as a result of which one observes

shift of the ignition temperature of the samples on the heated plate.

The mechanism of the effect of iron oxide is evidently different. It is known [5] that it is much weaker than \(CuO\) and \(MnO_2\) in catalyzing the oxidation of ammonia (moreover, it is an industrial catalyst for its synthesis). Therefore, the inhibition of the AP burning rate by the additive \(Fe_2O_3\) can be explained by inhibition of processes in the gas phase. It should be noted that \(MnO_2\), at plate temperatures of about \(800^\circ\), begins to exert a retarding effect. These results indicate that gas-phase reactions during combustion determine the rate of the whole process. The presence of additives changes the course of these reactions in such a way that the burning rate may either increase or decrease.

The strong acceleration by the \(Fe_2O_3\) additive of pyrolysis in the low-temperature region may be considered a consequence of catalysis of processes in the condensed phase, although it is still unclear which ones.

CONCLUSIONS

1. The effect of the additives \(CuO\), \(MnO_2\), and \(Fe_2O_3\) on the pyrolysis rate of ammonium perchlorate by a heated plate has been studied.

2. An explanation is given for the reasons for the change in the character of the influence of the additives at different plate temperatures.

LITERATURE

1. V. I. Bratinskii, G. V. Ivanov, N. I. Odishev. On the question of the mechanism of the influence of catalysts on the burning rate of condensed mixtures. Report in the present collection.

2. Chaiken R. F., Andersen W. H. J. Chem. Phys. 1960, 32, 141–146.

3. Hansel, M. K.-Alevi. Raketnaya tekhnika i kosmonavtika, 1966, No. 5, pp. 94–102.

4. V. F. Komarov, V. V. Boldyrev, V. K. Zhuravlev, G. V. Ivanov. Kinetics and Catalysis, 1967, vol. VII, issue 2, p. 817.

5. I. M. Morozov, L. I. Luk’yanova, N. N. Tsykin. Kinetics and Catalysis, 1966, vol. VII, issue 1, p. 172.